1. Field
The present invention relates generally to exploitation of clathrate reservoirs and more particularly to improving recoverability of clathrate reservoirs.
2. Background
Clathrates are substances in which one or more molecules of one or more compounds or elements (the guest(s)) fills one or more cavities within the crystal lattice of another compound (the host). Clathrates in which the crystal lattice is formed from water molecules are commonly called hydrates. Aspects of the present invention generally relate to all types of clathrates where the guest molecule(s) are one or more types of gasses, henceforth called gas clathrates. For the purposes of the present invention the term “clathrate(s)” should be understood to refer to all types of gas clathrates. In the field of hydrocarbon exploration and development, clathrates of interest are generally clathrates in which the guests are one or more hydrocarbon gasses and the hosts are water molecules. These are also sometimes called natural gas hydrates. They can be found in low temperature and/or high pressure environments, including, for example, deepwater and permafrost areas.
Clathrate reservoirs are classified according to a three class system. Class I reservoirs are clathrates underlain by and in fluid communication with a free gas reservoir. Class II reservoirs are clathrates underlain by and in fluid communication with a mobile aquifer reservoir. Class III reservoirs are clathrates underlain by a relatively impermeable layer. Class I reservoirs are in general considered to be relatively easy to produce hydrocarbons from, for example by drilling one or more production wells through the clathrate reservoir and into the free gas reservoir. By this method, the free gas reservoir reduces in pressure as it is produced, and this pressure drop eventually causes a pressure drop in the overlying clathrate reservoir to the extent that the clathrate reservoir is no longer in the phase stability envelope for the particular type of clathrate and dissociation (separation of the clathrate into water and gas(ses) commences. The released gas in effect recharges the underlying free gas reservoir, prolonging production from that reservoir. Unfortunately, Class I reservoirs are relatively rare. In general, Class II reservoirs are considered to be much more difficult to produce hydrocarbons from because the mobile aquifer acts to keep pressure in the overlying clathrate reservoir relatively high and interfere with or prevent dissociation. Class II reservoirs are relatively common. Class III reservoirs, like Class I reservoirs are in general considered to be relatively easy to exploit (for example, see U.S. Pat. No. 7,537,058 describing production from Class III reservoirs). The inventor has determined that it may be useful to convert Class II reservoirs into Class III reservoirs to improve the ability to produce hydrocarbons therefrom.